Shoot a wedding ( a once in a lifetime event for most people ) with the best gear you have, not as a pet project for yourself.

I would use the 6D and the 50 f1.4. Rent a 24-70 f2.8 and a 70-200 f4 L IS or 70-200 f2.8. And a flash if you don't have one.

By all means bring a film camera ( any EOS will do - think film choice instead ), but don't experiment with a wedding that's not yours to play with. I

You mentioned perhaps shooting 25% of the wedding on film. Think 5% and make it the least critical parts. Double every shot you take with film on digital.

Interestingly enough, there is a growing movement of wedding photographers who shoot weddings entirely on film. They charge a fortune, are usually fully booked a year out, and the shots (usually MF) look gorgeous.

For the OP, they said this: "My buddy is flying me back from China to shoot her wedding in a month. She's giving me a bit of cash which I will throw towards a long-overdue film camera, as she appreciates film."

No harm in shooting some of the wedding on film if the bride enjoys and appreciates film.

My last number of years of wedding shooting I was shooting the majority with film. My clients liked the idea of having shots on everything from Holgas and 35mm to 4x5. Funny how some people think shooting with film is almost dangerous.

Ha! It's probably because "do everything for them" digital cameras have made some of them dangerously inept. They can't imagine "flying blind."